Power off delay circuit and method, and audio system with power off delay

ABSTRACT

A power off delay circuit includes a switch connected between an external power input terminal and an internal power supply terminal, a capacitor connected to the internal power supply terminal, and a hysteresis comparator to switch the switch according to the voltages of the external power input terminal and the internal power supply terminal. During on-time of the switch, the external power input terminal is connected to the internal power supply terminal and the capacitor can be charged by the external power source. When the switch is off, the capacitor provides electric power for an internal circuit. Application of the power off delay circuit to an audio system may eliminate the turn-off pops of the audio system.

REFERENCE TO RELATED APPLICATION

This Application is being filed as a Divisional application Ser. No.12/818,539, filed 18 Jun. 2010, currently pending.

FIELD OF THE INVENTION

The present invention is related generally to an audio system and, moreparticularly, to a power off delay circuit and method for an audiosystem.

BACKGROUND OF THE INVENTION

Conventionally, to avoid pop generation by a loudspeaker of an audiosystem at power on or power off, an audio mute integrated circuit (IC)is adopted to eliminate pops. At power off, however, the audio mute IClacks a sufficient supply voltage to support proper operation of itsinternal circuit, and is thus hard to maintain its mute function. Tosolve this problem, the supply voltage of the audio mute IC must begiven with a longer sustained interval of time at power off so thatafter the power off, the internal circuit of the audio mute IC can stillwork normally for a period of time for the audio mute function to comeinto play, and the output signal of the audio system can still beproperly maintained for a period of time.

U.S. Pat. No. 5,778,238 inserts a P-N junction diode between an externalpower source and a capacitor to charge the capacitor, so that thecapacitor can provide energy for a low-voltage detector when the powersource is removed, and thereby turns on a MOSFET to discharge a delaycapacitor at the input terminal of a power on reset (POR) circuit. Thiswill prevent the delay capacitor from providing a shorter delay time atthe next power on. However, the diode connected between the externalpower source and the internal circuit leads to an additional voltagedrop, and thus decreases the margin of the working voltage of theinternal circuit. Moreover, the voltage outputted from the diode willfluctuate with the external supply voltage.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a power off delaycircuit and method for an audio system.

An object of the present invention is to provide an audio system withpower off delay.

According to the present invention, a power off delay circuit for anaudio system includes an external power input terminal, an internalpower supply terminal, and a switch connected therebetween. A hysteresiscomparator compares the voltages of the external power input terminaland the internal power supply terminal, thereby turns on the switch toconnect the external power input terminal to the internal power supplyterminal when the voltage of the internal power supply terminal is lowerthan that of the external power input terminal, to charge a capacitorconnected to the internal power supply terminal, and turns off theswitch to disconnect the external power input terminal from the internalpower supply terminal when the voltage of the internal power supplyterminal is higher than that of the external power input terminal.

According to the present invention, a power off delay method for anaudio system includes monitoring the voltages of an external power inputterminal and an internal power supply terminal and according thereto,hysteretically switching a switch connected between the external powerinput terminal and the internal power supply terminal. When the voltageof the internal power supply terminal is lower than that of the externalpower input terminal, the switch is turned on to charge a capacitorconnected to the internal power supply terminal. When the voltage of theinternal power supply terminal is higher than that of the external powerinput terminal, the switch is turned off.

According to the present invention, an audio system includes an audioline, a drive transistor connected to the audio line, and a power offdelay circuit connected to the drive transistor. The power off delaycircuit has an external power input terminal, an internal power supplyterminal, and a capacitor connected to the internal power supplyterminal. When the voltage of the internal power supply terminal islower than that of the external power input terminal, the capacitor ischarged. At power off, the capacitor supplies a current to the drivetransistor to pull down the voltage of the audio line.

Preferably, a PMOSFET is used as the switch to reduce the voltage dropthereacross as much as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsof the present invention taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of an audio system using a power off delaycircuit of the present invention;

FIG. 2 is an embodiment of the power off delay circuit according to thepresent invention;

FIG. 3 is an embodiment for the switch and the hysteresis comparatorshown in FIG. 2;

FIG. 4 is a diagram showing the I-V curves of a diode and a MOSFET;

FIG. 5 is a waveform diagram of an external supply voltage and aninternal supply voltage in the embodiment shown in FIG. 3; and

FIG. 6 is a diagram of comparison of an external supply voltage and aninternal supply voltage.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of an audio system using a power off delaycircuit of the present invention, in which a POR IC 10 is connected toan external power source Vcc, an external capacitor C, and a pluralityof drive transistors M1-MN, each of which is connected to a loudspeaker12 through an audio line 11. When the POR IC 10 detects an abnormalcondition from the external supply voltage Vcc, e.g. power down, a loadcurrent I_load is provided to the drive transistors M1-MN from electriccharges stored by the capacitor C to pull down the voltage of the audioline 11 to zero, to avoid pop generation. The POR IC 10 includes a poweroff delay circuit according to the present invention, which delays thetiming of internal power off of the POR IC 10 so that the POR IC 10 canstill maintain an enough load current I_load for a period of time afterthe power down of the external power source Vcc. As shown in FIG. 2, apower off delay circuit 14 includes the external capacitor C connectedto an internal power supply terminal V_(DD), a switch 16 connectedbetween an external power input terminal Vcc and the internal powersupply terminal V_(DD), and a hysteresis comparator 18 configured toswitch the switch 16 according to the voltages Vcc and V_(DD). Thehysteresis comparator 18 has a first input terminal connected to theexternal power input terminal Vcc, a second input terminal connected tothe internal power supply terminal V_(DD), and an output terminalconnected to the switch 16 for providing a control signal S1 to switchthe switch 16. In a first state, the switch 16 is on to connect theexternal power input terminal Vcc to the internal power supply terminalV_(DD) so that the capacitor C can be charged by the external powersource Vcc. In a second state, the switch 16 is off to disconnect theexternal power input terminal Vcc from the internal power supplyterminal V_(DD), and the capacitor C provides electric power foroperation of an internal circuit 20. Through hysteretically controllingconnection and disconnection between the external power input terminalVcc and the internal power supply terminal V_(DD), the internal supplyvoltage V_(DD) can be maintained stable. The capacitance of thecapacitor C defines the delay time of the power off delay circuit 14,i.e., the period of time for sustaining proper operation of the internalcircuit 20 by the power off delay circuit 14. In this embodiment, thecapacitor C is disposed outside the POR IC 10 to facilitate adjustmentof its capacitance to optimize the delay time. In other embodiments, thecapacitor C may also be disposed inside the POR IC 10 depending onsystem requirements.

FIG. 3 is an embodiment for the switch 16 and the hysteresis comparator18, in which the switch 16 includes a PMOSFET P1 connected between theexternal power input terminal Vcc and the internal power supply terminalV_(DD), under control of the control signal S1, and a voltage switchingcircuit having a PMOSFET P2 and a resistor R_(WELL) connected betweenthe external power input terminal Vcc and the internal power supplyterminal V_(DD). The PMOSFET P2 is connected between the external powerinput terminal Vcc and the well that the PMOSFET P1 is formed thereon,and the resistor R_(WELL) is connected between the internal power supplyterminal V_(DD) and the well that the PMOSFET P1 is formed thereon. ThePMOSFET P1 is used as the switch 16 in order to decrease the voltagedrop across the switch 16 as much as possible. The PMOSFET P2 and theresistor R_(WELL) are configured with a switching well architecture forconnecting the well that the PMOSFET P1 is formed thereon to the highestvoltage to improve the capability against latch-up. In this embodiment,the PMOSFET P1 defines the voltage drop between the external power inputterminal Vcc and the internal power supply terminal V_(DD) when theswitch 16 is on, the PMOSFET P2 and the resistor R_(WELL) are configuredto switch the voltage of an N-well between different ones, depending onconnection to either the external power input terminal Vcc or theinternal power supply terminal V_(DD). When the external supply voltageVcc is higher than the internal supply voltage V_(DD), the hysteresiscomparator 18 turns on the PMOSFETs P1 and P2 to connect the N-well tothe external power source Vcc through the PMOSFET P2 so that the base(i.e., the N-well) of the PMOSFET P1 is connected to a high voltageterminal Vcc. When the external supply voltage Vcc is lower than theinternal supply voltage V_(DD), the PMOSFETs P1 and P2 are turned off bythe hysteresis comparator 18 so that the base of the PMOSFET P1 isconnected by a parasitic resistor R_(WELL) to a high voltage terminalV_(DD). By switching the voltage of the N-well, the PMOSFET P1 operatesjust like a switch element. FIG. 4 is a schematic view of comparing theeffect of the present invention against that of the prior art, in whichΔV on the horizontal axis represents the voltage drop across the switch16, the vertical axis represents the current of the switch 16, the curve22 is the current-voltage (I-V) curve of the PMOSFET P1, and the curve24 is the I-V curve of a diode. The PMOSFET P1 operates as a switchelement with a voltage difference ΔV of about 0.1V existing thereacross,which is lower than the on-state voltage drop V_(DIODE) (about 0.6 V) ofthe diode, so the voltage drop between the external power input terminalVcc and the internal power supply terminal V_(DD) is reduced;furthermore, the internal supply voltage V_(DD) (=Vcc−ΔV) is higher thanthe internal supply voltage (=Vcc−V_(DIODE)) of the diode, so themarginal of the working voltage of the internal circuit 20 is increasedby about 0.5 V. On the other hand, the curve 22 has an increasing slope

Slope=1/Ron,   [Eq-1]

where Ron is the on-resistance of the PMOSFET P1. Increasing the size ofa PMOSFET will reduce its on-resistance Ron and thereby have a steeperincreasing slope Slope of the curve 22.

Referring back to FIG. 3, the hysteresis comparator 18 includes a pairof input transistors M1 and M2, the gate of the input transistor M1being connected to the external power input terminal Vcc, a bias currentsource I_(BIAS) connected to the input transistors M1 and M2, ahysteresis resistor R_(HYS) connected between the second input terminalof the hysteresis comparator 18 and the gate of the input transistor M2,and a hysteresis current source I_(HYS) connected in series to theresistor R_(HYS) to provide a current flowing through the resistor RHYSto establish a voltage drop to define the hysteresis band ΔH of thehysteresis comparator 18. Preferably, an initial-state setting resistorR_(INI) is connected to the output terminal of the hysteresis comparator18 to preset the output signal S1 of the output terminal to a logic lowlevel so that the PMOSFET P1 is preset to on state. Referring to FIG. 5,the waveform 26 represents the external supply voltage Vcc, the waveform28 represents the internal supply voltage V_(DD), and the level 30represents a standby power of the external power source Vcc, usually 3.3V or 5 V. After power on, the external supply voltage Vcc rises from 0Vto a rated value, during which process the internal supply voltageV_(DD) also rises accordingly because the switch 16 is on. Due to thehysteresis of the hysteresis comparator 18, the switch 16 is turned offat a later time t1 until the internal supply voltage V_(DD) falls belowa threshold value, e.g. at time t2, and then the hysteresis comparator18 turns on the switch 16 again so that the capacitor C is charged bythe external power source Vcc to pull up the internal supply voltageV_(DD). At time t3, the switch 16 is turned off by the hysteresiscomparator 18 again to cause the internal supply voltage V_(DD) to startto fall down again. After the external supply voltage Vcc falls belowthe standby power 30, the capacitance of the capacitor C determines thedecreasing slope of the internal supply voltage V_(DD) as

R _(SW)=decreasing slope of Vcc (V/s),   [Eq-2]

C>I_load/R _(SW).   [Eq-3]

For example, if I_load=5 mA, R_(SW)=5V/1 ms=5 K(V/s), then

C>5 mA/5 KV/s=1 μF.

If I_load=20 mA, R_(SW)=5V/10 ms=0.5 K(V/s), then

C>20 mA/0.5 KV/s=40 μF.

As shown by the dash line 32 in FIG. 5, when the capacitor C has agreater capacitance, the decreasing slope of the internal supply voltageV_(DD) becomes flatter.

Referring to FIG. 6, after power on, when the external supply voltageVcc rises to a cut-in voltage Vr of the PMOSFET P1, the PMOSFET P1 isturned on and as a result, the internal supply voltage V_(DD) jumps to alevel that is lower than the external supply voltage Vcc by about 0.1Vand then rises with the external supply voltage Vcc. After the externalsupply voltage Vcc reaches the rated value, the internal supply voltageV_(DD) reaches Vcc later due to hysteresis. Afterwards, the internalsupply voltage V_(DD) is maintained at around VCC by the hysteresiscomparator 18, and the ripple of the internal supply voltage V_(DD) isdetermined by the hysteresis band ΔH. During this process, the switch 16is switched on and off repeatedly by the control signal S1, withduration T of each turn-off period depending on the load current I_loadand the hysteresis band AH. By selecting an appropriate hysteresis bandΔH, the switching frequency of the switch 16 and, therefore, theswitching power loss can be reduced.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

1. An audio system, comprising: an audio line; a drive transistorconnected to the audio line; and a power off delay circuit connected tothe drive transistor, the power off delay circuit having an externalpower input terminal, an internal power supply terminal, and a capacitorconnected to the internal power supply terminal, wherein the capacitoris charged during the internal power supply terminal has a voltage lowerthan that of the external power input terminal, and provides a currentto the drive transistor to pull down a voltage of the audio line atpower off.
 2. The audio system of claim 1, wherein the power off delaycircuit comprises: a switch connected between the external power inputterminal and the internal power supply terminal; and a hysteresiscomparator having a first input terminal connected to the external powerinput terminal, a second input terminal connected to the internal powersupply terminal, and an output terminal connected to the switch forproviding a control signal to switch the switch, wherein the switch ison in a first state to connect the external power input terminal to theinternal power supply terminal, and is off in a second state todisconnect the external power input terminal from the internal powersupply terminal.
 3. The audio system of claim 2, wherein the switchcomprises a MOSFET connected between the external power input terminaland the internal power supply terminal, controlled by the controlsignal.
 4. The audio system of claim 2, wherein the switch comprises: aPMOSFET formed on a well, connected between the external power inputterminal and the internal power supply terminal, controlled by thecontrol signal; and a voltage switching circuit connected to the well toswitch a voltage of the well.
 5. The audio system of claim 4, whereinthe voltage switching circuit comprises: a second PMOSFET formed on thewell, connected between the external power input terminal and the well,operative to connect the external power input terminal to the well inthe first state; and a resistor connected between the internal powersupply terminal and the well, for applying a voltage from the internalpower supply terminal to the well in the second state.
 6. The audiosystem of claim 5, wherein the resistor comprises a well resistorestablished by the well.
 7. The audio system of claim 2, wherein thehysteresis comparator comprises an initial-state setting resistorconnected to the output terminal of the hysteresis comparator, forsetting an initial logic state of the control signal.
 8. The audiosystem of claim 2, wherein the hysteresis comparator comprises: a firstinput transistor having a gate connected to the external power inputterminal; a second input transistor; a hysteresis resistor connectedbetween the second input terminal of the hysteresis comparator and agate of the second input transistor; and a hysteresis current sourceconnected in series to the hysteresis resistor; wherein the hysteresisresistor establishes a voltage drop to determine a hysteresis band ofthe hysteresis comparator.
 9. The audio system of claim 1, wherein thecapacitor has a capacitance defining a delay time of the power off delaycircuit.